1. Field of the Invention
The present invention relates to a method for controlling a coordinate positioning machine to scan a workpiece.
Such a machine, (e.g. a machine tool or coordinate measuring machine) typically comprises a head, movable in three directions (X,Y,Z) relative to a bed, and a scale, together with a scale-reader for measuring the displacement of the head relative to a datum in each of the X,Y and Z directions. The head of the machine carries a measuring probe for measuring the distance between the head and a surface; the measuring probe may be an optical probe or a touch probe. An optical probe directs a beam of light at the surface, and determines from the reflected beam the position of the surface relative to the head. A contact or touch measuring probe (also known as an analogue touch probe) resiliently supports a stylus on the head in such a manner that the stylus is movable relative to the head in the X,Y,Z directions. Transducers in the probe measure the displacement of the stylus relative to the head in each of the X,Y and Z directions.
It is possible using such a machine, to scan the profile of a workpiece. This is done by moving the head of the machine to bring the probe adjacent the surface to within a range of distance over which the probe may measure the position of the surface (known as the measuring range), and then driving the head of the machine over the surface along a path whose profile is to be measured. When driving the probe over the surface, the probe must be kept within the measuring range (a) in order to collect data, and (b) to prevent collision damage to the machine. Automatically controlling the motion of the head of the machine relative to a surface within the constraints set out above can be difficult when the workpiece contour is not known in advance.
2. Description of Related Art
One prior art method (known from U.S. Pat. No. 4,769,763) of achieving this with a touch probe is to drive the head of the machine in a direction determined from the instantaneous direction of deflection of the stylus from its rest position. Friction between the stylus and the surface makes this technique inaccurate since deflection of the stylus is not perpendicular to the instantaneous path direction.
A prior art method of controlling a machine with an optical probe is shown in FIG. 1. An optical probe 10 directs a beam of light 12 at a surface 14. The beam 12 is reflected by the surface 14, and a position sensitive detector in the probe detects the reflected beam and thus the position of the surface. Such a probe is known from EP 0235235. Movement of the machine is controlled by determining a straight line 16 passing through two previously measured points Q.sub.1,Q.sub.2, and predicting the position of the next demanded point Q.sub.3 as lying on this line.